Yeah, but he just turned 17. That's incredibly young no matter how good he is. Messi only really got a first team role for Barcelona once he turned 18 (the summer before the 05/06 season) and he made 25 appearances in all competition that season. That timeline would basically him being the 2nd half of next season having an impact on the squad, and again, only really establishing himself in the first team as a bigger role until the 2020/21 season. He turns 19 in that season. Mbappe only turned into a starter and a key player for Monaco after he turned 18, and he's a once a generation talent. Rooney started off early because of how developed he was physically. Ronaldo was 18 and a half when he joined United. Sancho right now for Dortmund is turning 19 in a few months. He only started getting a bigger role with Dortmund this season, when he was again, 18.5. Greenwood is a year and a half from that age.
Of course he's incredibly talented, but people need to realize that 2 months ago he was 16 years old. People progress quickly at a young age, but even as a very promising youngster who develops quickly, you can't expect anything out of him til the 2nd half of next season as a best case scenario, when he is a few months past his 18th birthday. How often do you see a freshly turned 17 year old literally do anything at a first team level? Simply getting a sub appearance and not being thrown around like a kid will be a positive.
This season is way too soon for him. Next season, more likely the second half of the season. He'll be 18 still by the time the 2020/21 season starts, don't forget.